Experiments in progress include an investigation of the utilization of microtubular protein in various structures of developing Xenopus embryos. Microtubular protein has been purified to 90% purity from Xenopus eggs and from oocytes. It has been labelled and injected into fertilized eggs and shown to be incorporated into spindles. Heterologous microtubular protein is not utilized. Experiments are underway to look biochemically at turnover and utilization of oocyte protein in later development. Experiments are also underway to study polymerization in vitro of microtubules and to study factors required for initiation in vivo of mitotic apparatus formation.